Miniaturized cantilever probe for scanning probe microscopy and fabrication thereof

ABSTRACT

Cantilever probes are formed from a multilayer structure comprising an upper substrate, a lower substrate, an interior layer, a first separation layer, and a second separation layer, wherein the first separation layer is situated between the upper substrate and the interior layer, the second separation layer is situated between the lower substrate and the interior layer, and wherein the first and the second separation layers are differentially etchable with respect to the first and the second substrates, the interior layer. The upper substrate is a first device layer from which a probe tip is formed. The interior layer is a second device layer from which a cantilever arm is formed. The lower substrate is a handle layer from which a handle, or base portion, is formed. Patterning and etching processing of any layer is isolated from the other layers by the separation layers.

FIELD OF THE INVENTION

The present invention relates generally to scanning probe microscopyand, more particularly, to a miniaturized cantilever probe structure andmethods of producing the same.

BACKGROUND OF THE INVENTION

Scanning probe microscopes (SPMs), such as the atomic force microscope(AFM), are instruments which typically use a sharp tip to characterizethe surface of a sample down to nanoscale dimensions. The term nanoscaleas used for purposes of this invention refers to dimensions smaller thanone micrometer. SPMs monitor the interaction between the sample and theprobe tip. By providing relative scanning movement between the tip andthe sample, surface characteristic data can be acquired over aparticular site on the sample, and a corresponding map of the site canbe generated. Because of their resolution and versatility, SPMs areimportant measurement devices in many diverse fields includingsemiconductor manufacturing, material science, nanotechnology, andbiological research.

The probe of a typical SPM includes a very small cantilever fixed to asupport (i.e., a handle) at its base and having a sharp probe tipextending from the opposite, free end. The probe tip is brought verynear to or into contact with a surface of a sample to be examined, andthe deflection of the cantilever in response to the probe tip'sinteraction with the sample is measured with an extremely sensitivedeflection detector such as an optical lever system as described, forexample, in Hansma et al. U.S. Pat. No. RE 34,489, or some otherdeflection detector such as strain gauges, capacitance sensors, etc. Theprobe is scanned over a surface using a high resolution three-axisscanner acting on the sample support, the probe, or a combination ofboth. The instrument is thus capable of measuring the topography orother surface properties or nanomechanical properties of the sample.Cantilever probes can be made from conductive material, enablingmeasurement of electrical properties.

SPMs may be configured to operate in a variety of modes, including modesfor measuring, imaging, or otherwise inspecting a surface, and modes formeasuring nanomechanical properties of a sample. In a contact modeoperation, the microscope typically scans the tip across the surface ofthe sample while maintaining a constant probe-sample interaction force.In an oscillation mode of operation, sometimes referred to as tappingmode, the tip of the SPM is oscillated while interacting with the sampleat or near a resonant frequency of the cantilever of the probe. Theamplitude or phase angle of this oscillation is affected by theprobe-sample interaction, and changes in the oscillation are sensed.

As the probe is scanned over the surface of the sample, a probepositioning control system monitors the interaction of the probe withthe sample surface such as, for example, deflection of the cantilever(in the case of contact mode), or changes in the oscillation amplitudeor phase angle (in the case of oscillating mode). The control systemadjusts the probe's position (or average position in the case ofoscillating mode) relative to the sample to maintain a constantprobe-sample interaction. The position adjustment thus tracks thetopography of the sample. In this way, the data associated with theposition adjustment can be stored, and processed into data thatcharacterizes the sample. This data can be used to construct an image ofthe inspected sample's surface, or to make certain measurements ofselected surface features (such as, for example, a height of thefeature).

The probe position adjustment is effected by a cantilever positioningactuator that is driven by a driving circuit. Various technologies forcantilever actuators are known, including piezoelectric and magnetictransducers. The driving circuit generates a probe positioning signal,and amplifies the probe positioning signal to produce a driving signalthat is applied to the actuator. The driving signal continuouslyrepositions the probe's separation distance from the sample to track anarbitrary topography of the sample's surface. Accordingly, the drivingsignal has a bandwidth from zero hertz to a frequency associated withthe maximum operating bandwidth of the SPM, which corresponds to themaximum speed at which the probe can track the topography of the surfaceof the sample.

Some of the more recent developments in SPM technology have focused onhigh-speed scanning that can provide scanning speeds at a video ratesuch that the sample can be observed in near real-time. This presents anumber of challenges. For one, the cantilever probe should have a highenough resonance frequency to enable scanning over an arbitrarytopography at a required video rate. Ideally, the cantilever should beas “fast” as possible so that larger areas can be scanned at the desiredrate. Resonance frequencies on the order of 1 MHz or more are desired.At the same time, the cantilever probe should provide maximumsensitivity in terms of deflection amount for a given interaction forcewith the sample. A more sensitive cantilever probe can be used tominimize the forces exerted on samples during measurement, therebyobtaining better characterization of a sample's true properties.

Unfortunately, there is a trade-off between speed and sensitivity incantilever probes. A softer, i.e., lower-spring-constant cantileverprobe that is more sensitive tends to have a lower resonancecharacteristic, thereby being less fast. Improvement in both propertiescan be obtained by scaling down the cantilever probe's dimensions.Accordingly, a cantilever probe that is shorter, narrower, and thinner,is desired.

Scaling down the cantilever probe dimensions presents its own host ofproblems. The generally preferred fabrication process is a batch processutilizing nanoelectromechanical systems (NEMS) techniques applied on awafer scale rather than on an individual basis. These techniques includethin film deposition and photolithography operations enabling themass-production of cantilever probes. As the smallest dimensions of thecantilever probe are reduced to the sub-micron scale, the conventionalfabrication processes become exceedingly difficult to control uniformly.This problem lies in the use of chemical etching and controlling theamount of etching of the thin membrane that is to become the cantileverarm.

Individually-fabricated high-performance probes can be made usingtechniques such as Electron Beam-Induced Deposition (EBID) of the probetip as a step separate from formation of the cantilever arm. However,these single-device techniques are not amenable to mass production.Probes produced in this manner can cost 1-2 orders of magnitude morethan mass-produced probes. Therefore, a solution is needed to enablebatch (wafer-scale) production of cantilever probes having similar orbetter characteristics than individually-produced probes. The batchprocess means producing multiple devices or AFM probes in parallel oneach wafer and processing one or more wafers simultaneously. Typicalmaterials used for cantilever probes include cantilever arms made fromsilicon or silicon nitride (Si_(x)N_(y)) film. Probe tips can be madefrom a variety of materials, though silicon is generally preferablybecause the tips can be made very sharp using relatively simple etchingtechniques and a thermal oxidation process in which a layer of silicondioxide, (SiO₂) is grown on the tip structure.

In the fabrication of a traditional cantilever probe made from a siliconcantilever arm with a silicon probe tip, the starting point is usually ablank silicon wafer. Cantilever arms are produced by reducing the blankwafer thickness in a certain patterned area, typically with a density of300 devices in a 4 inch wafer. Such a wafer usually has variation of thethickness in some areas, typically +/−1 micron for a 300 micron-thickwafer. As the etching proceeds to produce the cantilever arm of 1 micronthickness, for example, etching must remove 299 microns of material a300-micron wafer. Due to the +/−1 micron uneven thickness of the wafer,the removal of 299 microns of material will produce 1 micron-thickcantilever arms in an area of 300-micron thickness, 2 micron-thickcantilever arms in the area of 301 micron thickness, and no cantileverarm at all in an area of 299-micron thickness. The cantilever armformation yield from the various processing steps of forming thecantilever arms, defined by cantilever arms with a thickness within adesired range, is only around 30% if the +/−1 um unevenness of the waferthickness is equally distributed throughout the wafer (in regions havingsizes exceeding the size of the cantilever arms). In an industrial-scaleproduction environment, such yield is unacceptable. It has been verydifficult to form a thin layer of silicon less than 1000 nm thick to beused as cantilever arms from a larger bulk material without eitherencountering a low production yield or large variation in cantilever armthickness from one batch to another even if the etch rate is controlledprecisely. In produced batches with relaxed tolerances, the thicknessvariation produces an exponentially greater variation in spring constantbecause spring constant is proportional to the third power of cantileverarm thickness.

Making cantilever probes with arms thinner than 1 micron is far moredifficult. One approach has been to utilize a chemically dissimilarmaterial, such as a thin film of silicon nitride (Si₃N₄) with an eventhickness by chemical vapor deposition (CVD) and pattern the depositedmaterial into a cantilever arm. In these conventional cantilever probesmade with silicon nitride cantilever arms and silicon tips, the nitridecantilever arm can be made thinner and with a uniform thickness than thecantilever arm of an all-silicon probe because etching of material toform the silicon tip and handle has virtually no effect on the nitridecantilever arm. In this case it is much easier to control cantileverthickness and size. However, such a process creates other challenges,namely, with using silicon to form a sharp tip by anisotropic etchingbecause there is no silicon material above the CVD-applied Si₃N₄ to formthe tip. Typically, a serial (non-batch) process such as electron beamdeposition is used to form a tip on a silicon nitride cantilever arm.

Other approaches have been proposed for batch processing, though withmixed results and limitations in scaling down the dimensions. Moreover,in batch processing to form the silicon tip and to pattern the nitridecantilever arm to its desired width dimensions and shape, conventionalprocessing has often required photolithography operations to beperformed on structures supported by only the thin nitride membrane.This results in breakage of around 10% of the cantilever arms of atypical batch, with another 10% being lost to other causes. Even withthis reduced yield, the thinnest practical cantilever arm that can bemade economically using the conventional process is around 200 nm.

More recent advances have introduced a buried layer technology in which,prior to cantilever probe formation, multi-layer structures are formedat the wafer level. In one type of structure called silicon-on-insulator(SOI), used in the process disclosed in Qingkai Yu, Guoting Qin, ChinmayDarne, Chengzhi Cai, Wanda Wosik, and Shin-Shem Pei, Fabrication ofShort and Thin Silicon Cantilevers for AFM with SOI Wafers, 126 Sensorsand Actuators A: Physical, Issue 2 (2006) pp. 369-374, a layer of oxideis grown on top of a first silicon wafer; then, a second silicon waferis bonded to the top of the oxide to form a single silicon-oxide-siliconwafer or slab, from which multiple cantilevers can be formed by batchprocessing. The buried oxide layer separates the device layer on whichthe silicon cantilever arm and probe tip are formed, from the handlelayer in which a large portion of silicon at the base of the cantileveris kept. This allows the handle layer to be etched using a chemical thatattacks silicon but not silicon dioxide, to release the cantileverwithout also etching into the device layer. However, the device layermust still be etched using a time-controlled process to control thecantilever arm thickness. Reduced etchant concentrations may be used toslow down the material removal process for greater control, but doing soreduces the processing throughput and increases the expense ofcantilever probe fabrication.

In U.S. Pat. No. 7,913,544, a fabrication process is proposed in which aburied oxide layer is used in the fabrication of a cantilever probe witha nitride film cantilever arm and a silicon tip. In this approach, theprobe tip is formed with a base pad. Thereafter, the nitride film isdeposited so that part of it binds to the silicon tip's base pad. Theresulting cantilever structure has the base pad and cantilever tip atthe free end of the cantilever arm. This approach can address theproblem of having to pattern the thin film nitride cantilever arm afterit has been released from the backside; however the base pad thicknessis difficult to control for the reasons discussed above. The size andperformance of the cantilever are also limited in this process due tochallenges with aligning the base pad and nitride cantilever arm, andhaving a cantilever structure in which the effective cantilever arm iscomposed of both, nitride film, and the part of the base pad leading upto the probe tip.

In U.S. Pat. No. 7,182,876, a buried nitride-oxide multi-layer structureis proposed as the starting point for batch cantilever probefabrication, with the nitride layer intended to be formed into thecantilever arm in later processing steps. The use of a buried nitridelayer allows formation of the probe tip directly over the cantileverarm; however, this approach requires protecting the probe tip when thehandle layer, made from the same material, is etched. Typical protocolfor protecting the probe tip involves depositing a protective coating ofnitride film over the tip, which resists the chemical etching of thehandle layer. However, the protective layer must then be removed, andthis removal etch will also etch the nitride film used for thecantilever arm. Therefore, the same problem of controlling the thicknessof the cantilever must be solved with this approach as well.

In general the SPM probe performance is gauged by the operatingbandwidth, which is proportional to the resonance frequency f of thecantilever, and level of force control which is inversely proportionalto the cantilever spring constant k. Maximizing f/k, or commonlyexpressed as f²/k, is regarded as optimizing of the probe performance.According to Sader et al., Calibration of Rectangular Atomic ForceMicroscope Cantilevers, Rev. Sci. Instrum. 70, 3967 (1999), incorporatedby reference herein, f²/k˜1/bhL, where L is the length, b is the widthand h is the thickness. Consequently reducing all the dimensions of thecantilever arm in proportion provides much improved performance. In theextreme case, as shown by Ando et al., A High-Speed Atomic ForceMicroscope for Studying Biological Macromolecules, PNAS 98, 12468 (2001)the cantilever size is reduced to 2 um×8 um×0.1 um for width, length andthickness respectively. However, scaling down the probe size presentsmajor challenge to batch processing for the many reasons discussedabove. In addition to the difficulty of forming a tip on a smallcantilever of the size reported in Ando et al., placement of the tip atthe small cantilever's free end is also very difficult because of thelithographic error. The SPM field has thus relied on expensive andserial processes to produce such small probes.

An additional demand in SPM applications is that the probes of a givenmodel type are substantially uniform from one to the next, meaning thespring constant variation is preferably smaller than 30%, resonancefrequency variation is smaller than 30%, and tip position and geometricvariation (tip height and apex radius variation) is less than 40%. Ithas been impractical to produce such probes in commercial quantities.

In view of the above, a solution to the numerous challenges ofminiaturizing silicon tip cantilevers made with either silicon ornitride cantilever arms, is needed.

SUMMARY OF THE INVENTION

Aspects of the invention provide a method that can produce smallcantilever probes using a batch process with autonomous control suchthat the cantilever probes have high performance in terms of resonantfrequency and spring constant, and such that batch-manufactured probeshave better uniformity than what has been previously possible in theart. One aspect of the invention is directed to a method forconstructing a multilayer structure to be used for scanning probemicroscope (SPM) cantilever fabrication. According to the method, afirst substrate and a second substrate are provided along with a firstseparation layer and a second separation layer. The first separationlayer and the first substrate are differentially etchable and the secondseparation layer and the second substrate are differentially etchable.An interior layer is provided to be situated between the firstseparation layer and the second separation layer. The interior layer isdifferentially etchable with respect to the first and the secondseparation layers. The multilayer structure is assembled, includingsituating the interior layer between the first separation layer and thesecond separation layer such that the interior layer is separated fromthe first substrate and the second substrate, respectively, by the firstseparation layer and the second separation layer.

In a related aspect of the invention, after assembling the multilayerstructure, material from the first substrate and the first separationlayer is selectively removed to expose a first surface of the interiorlayer while maintaining bonding of a second surface of the interiorlayer that opposite the first surface, with another layer. Thereafter,portions of the interior layer are removed to form a cantilever armtherefrom. thereafter, material from the second substrate and the secondseparation layer is selectively removed to expose the second surface ofthe interior layer, thereby releasing the cantilever arm. A process suchas this one is capable of batch-producing cantilever probes having aspring constant of between 0.1 and 1 N/m and a combined cantilever armand tip with a resonance frequency of between 100 kHz and 10 MHz.

A cantilever probe for use with a scanning probe microscope (SPM)according to another aspect of the invention includes: a base portionformed from a bulk semiconductor material; a cantilever arm having aproximal end situated over the base portion and a distal end protrudingbeyond the periphery of the base portion; a lower separation layersituated between the base portion and the proximal end of the cantileverarm, the lower separation layer being differentially etchable relativeto the cantilever arm and the base portion; a probe tip situated over aportion of the distal end of the cantilever arm; and an upper separationlayer situated between the probe tip and the cantilever arm, the upperseparation layer being differentially etchable relative to thecantilever arm.

Another aspect of the invention is directed to a mass-produced batch ofcantilever probes for use with a scanning probe microscope. The batchincludes a plurality of cantilever probes. Each probe includes a baseportion formed from a bulk semiconductor material; a cantilever armhaving a proximal end situated over the base portion and a distal endprotruding beyond the periphery of the base portion and a thickness ofbetween 30 and 300 nm; and a silicon probe tip situated over a portionof the distal end of the cantilever arm. The unsorted variability of thecantilever arm thickness within the batch of cantilever probes isbetween +/−5% and wherein the production yield within the batch ofcantilever probes (defined as the ratio of non-broken cantilever arms)is greater than 90%.

In another aspect of the invention, a process of simultaneouslyfabricating a batch of cantilever probes, with each probe comprising ahandle portion, a cantilever arm, and a probe tip, includes thefollowing operations:

-   -   obtaining a multi-layer composite wafer that includes a first        layer, a second layer, and a third layer, wherein the first        layer includes material for the handle portion of every        cantilever probe of the batch, the second layer includes        material for the cantilever arm of every cantilever probe of the        batch, and the third layer includes material for the probe tip        of every cantilever probe of the batch;    -   removing excess material from the third layer with a first set        of etch operations to simultaneously form a plurality of        cantilever tips of the plurality of cantilever probes;    -   removing excess material from the second layer with a second set        of etch operations to simultaneously form a plurality of        cantilever arms of the plurality of cantilever probes; and    -   removing excess material from the first layer with a third set        of etch operations to simultaneously form a plurality of handle        portions of the plurality of cantilever probes;    -   wherein removal of the excess material from each of the first,        second, and third layers results in formation of individual        cantilever probes having dimensions between 5 and 30 microns in        length, between 2 and 15 microns in width, and between 30 and        300 nanometers in thickness, and wherein the thickness of the        cantilever arms is not dependent on a duration of the first,        second, and third etch operations.

In yet another aspect of the invention, a cantilever probe includes ahandle portion, a cantilever arm, and a probe tip. The cantilever probeis produced by process of simultaneously fabricating a batch ofcantilever probes, as stated above.

A cantilever probe according to still another aspect of the invention isproduced by process of simultaneously fabricating a batch of cantileverprobes, the process comprising:

-   -   obtaining a multi-layer composite structure that includes a        first layer, a second layer, and a third layer, wherein the        first layer includes material for the probe tip structure of        every cantilever probe of the batch, the second layer includes        material for the cantilever arm of every cantilever probe of the        batch, and the third layer includes material for the handle        portion of every cantilever probe of the batch;    -   for each cantilever probe of the batch:        -   performing a high-aspect-ratio anisotropic etch to remove a            portion of the first layer and expose an orthogonal surface            of the first layer that is oriented orthogonally to the            layers of the multilayer structure;        -   forming an orthogonal protective layer over the exposed            orthogonal surface;        -   selectively removing additional material from the first            layer with an isotropic etch to form a probe tip structure            having a sharp point facing away from the second layer,            wherein the orthogonal protective layer preserves the            orthogonal surface during the isotropic etch; and        -   thereafter, removing the orthogonal protective layer to            expose the orthogonal surface of the first layer, wherein            the orthogonal surface includes an apex of the probe tip            structure.

Another aspect of the invention is directed to a wafer having a batch ofpartially-formed cantilever probes fabricated simultaneously thereuponfor use with a SPM. The wafer includes a plurality of cantilever probesto be released from the wafer in a subsequent operation, each one ofwhich has a base portion formed from a bulk semiconductor material, acantilever arm having a proximal end situated over the base portion anda distal end protruding beyond the periphery of the base portion and athickness of between 30 and 300 nm, and a silicon probe tip situatedover a portion of the distal end of the cantilever arm. The cantileverarm has a spring constant of between 0.1 and 1 N/m and a combinedcantilever arm and tip with a resonance frequency of between 100 kHz and10 MHz.

A further aspect of the invention is directed to an improved cantileverprobe comprising a handle portion, a cantilever arm having a proximalend and a distal end, with the proximal end situated over the handleportion and the distal end protruding beyond a periphery of the handleportion, and a probe tip structure situated near the distal end of thecantilever arm. In one type of embodiment, the improvement includes thecantilever arm having a length dimension between the proximal and thedistal ends, a width dimension, and a thickness dimension, wherein in areference plane defined by the length dimension and the width dimension,the cantilever arm has a paddle profile that includes a face portion atthe distal end and a neck portion between the face portion and the baseportion, the neck portion having a smaller width dimension than the faceportion.

In another type of embodiment, the improvement includes the cantileverarm having a length dimension between the proximal and the distal ends,a width dimension, and a thickness dimension, wherein in a referenceplane defined by the length dimension and the width dimension, thecantilever arm includes a neck portion and a shoulder portion, whereinthe shoulder portion is situated at the proximal end of the cantileverarm and protrudes in the distal direction beyond a periphery of the baseportion, and wherein the neck portion has a substantially smaller widthdimension than the shoulder portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a top-level functional diagram illustrating a typical SPMsystem in which aspects of the present invention are incorporated.

FIGS. 2A-2C generally illustrates amplitude spectra corresponding toactuator motions for topography tracking, oscillation mode, and coarseprobe positioning, respectively.

FIGS. 3A-3D are diagrams illustrating various actuator drivingconfigurations for piezo stack actuators.

FIG. 4A is a schematic diagram illustrating some of the major parts ofan exemplary cantilever probe according to certain aspects of theinvention.

FIG. 4B is a diagram illustrating the beneficial functionality of ashoulder structure according to one type of embodiment.

FIGS. 5, 6A, and 6B illustrate exemplary multilayer structures fromwhich cantilever probes can be formed using batch processing to producea silicon-on-insulator (SOI) probe, a silicon-on-silicon (SOS) probe,and an SOS probe in which the probe tip is electrically connected to thecantilever arm according to various embodiments.

FIGS. 7A-7E illustrate, step-by-step, an exemplary process of formationof the multilayer structure of FIG. 5 according to one embodiment.

FIGS. 8A-8E illustrate an exemplary process according to variousembodiments, for forming the multilayer structure of FIG. 6A.

FIGS. 8E′-8H′ illustrate a variation of the processing of FIGS. 8D-8E toform the multilayer structure of FIG. 6B according to one embodiment.

FIGS. 9A-9N illustrate an exemplary process according to one embodimentfor batch fabrication of a silicon-on-insulator (SOI) cantilever probestarting with multilayer structure of FIG. 5.

FIGS. 10A-10O illustrate a batch process according to one embodiment forforming a SOI cantilever probe with a self-aligned probe tip that isaligned at the distal end of the cantilever arm.

FIGS. 11A-11L illustrate an exemplary batch process for fabricating acantilever probe having a very thin silicon cantilever arm and siliconprobe starting with the multilayer structure of FIG. 6A according to oneembodiment.

FIGS. 12A-12L illustrate an exemplary batch process for forming aconductive cantilever probe starting with the multilayer structure ofFIG. 6B according to one embodiment.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a top-level diagram illustrating a portion of a typical SPMsystem 100. System 100 includes cantilever 102 that holds probe 104.Probe 104 is used to inspect sample 106 having surface 108. Surface 108has a certain topography, which is the subject of the inspection incertain applications. More generally, for samples that do not have majorsurfaces defined by corresponding aspect ratios, the topography of thesample can similarly be inspected by the SPM. The term topography isdefined herein as a representation of a three-dimensional profile of asample or of a portion of a sample, including, but not limited to,surface features of a sample.

The inspection is accomplished by situating probe 104 relatively tosurface 108 by moving either cantilever 102, sample 106, or both, inorder to establish a detectable interaction between surface 108 andprobe 104. Probe 104 is scanned over or across the sample while probe104 tracks the topography of sample 106, such as, for example, thetopography of surface 108. Tracking of the topography includes, withoutlimitation, following features present on the sample, such as, forexample, lines, troughs, walls, corners, cavities, protrusions, and thelike.

In one embodiment, as illustrated in FIG. 1, SPM system 100 includes anactuator subsystem in which actuator 110 adjusts the probe-sampleinteraction by moving cantilever 102 relative to surface 108 to producemotion 202 as depicted in FIG. 2A. FIG. 2A illustrates the amplitudespectrum of motion 202, in which motion 202 has an amplitude A and abandwidth based on rolloff frequency f₁. In related embodiments,actuator 110 may move sample 106 relative to cantilever 102, or actuator110 may move both sample 106 and cantilever 102. The relative motion ofcantilever 102 and sample 106 can be perpendicular or oblique to surface108, and can include translational or rotational motion components. Forthe sake of simplicity, the relative repositioning of sample 106 andcantilever 102, which adjusts the level of probe-sample interactionshall be termed herein as motion in the z direction, or along the zaxis.

Persons skilled in the relevant arts will appreciate that SPM system 100can be constructed such that the mechanical structure, including thecouplings between probe 104 and sample 106, reduce or avoid damping,resonating, or otherwise interacting with the relative motion betweenthe probe and the sample. For instance, actuator 110 may be rigidlymounted to a chassis to which sample 106 is also rigidly mounted.

Additional mechanical systems 112 may also be provided that wouldinclude actuators to move cantilever 102 or sample 106, or both, suchthat the relative motion of probe 104 and surface 108 is along the planethat is generally parallel to surface 108. For simplicity, this motionshall be termed motion in the x-y directions or along the x- or y-axes.This motion in the x-y directions facilitates the scanning of probe 104over surface 108. As probe 104 is scanned over sample 108, actuator 110adjusts the relative positioning of cantilever 102 and surface 108 toproduce motion 202 to maintain a generally constant level ofprobe-sample interaction, which results in probe 104 tracking thetopography of surface 108. In oscillating mode systems, the probe-sampleinteraction can be averaged over the oscillation cycle to produce acorresponding scalar representing the level of probe-sample interactionto be maintained as probe 104 is scanned over surface 108.

Additional mechanical systems 112 can facilitate the oscillatory motionof cantilever 102 for those embodiments that operate in oscillatingmode. FIG. 2B illustrates the amplitude spectrum of oscillating motion204 in accordance with oscillating mode. Oscillating motion 204 has anamplitude B that is substantially smaller than amplitude A. Although theoscillating motion 204 is typically in the z-direction and affects theprobe-sample interaction at different points along the oscillationcycles, the oscillating motion is not generally used to track thetopography of surface 108. Oscillating motion 204 has narrow bandwidthcentered at a frequency f₂ at or near the resonant frequency of thecantilever/probe mechanism, which can be much higher than frequency f₁.This relatively narrow bandwidth prevents motion 204 from moving probe104 to track the arbitrary topography of surface 108. The bandwidth ofthe z-axis motion includes zero hertz (i.e. DC). Amplitude A oftopography-tracking motion 202 has a significantly greater displacementthan the amplitude B of the oscillation motion 204. In one embodiment,for instance, topography-tracking motion 202 has a displacement of atleast 1 micron. In another embodiment, motion 202 has a displacement ofat least 10 microns.

Further, additional mechanical systems 112 can include a coarseadjustment along the z axis for fast engagement and disengagement ofprobe 104 and surface 108. FIG. 2C illustrates the amplitude spectrum ofcoarse adjustment motion 206. Coarse adjustment motion 206 has a anamplitude C, which provides a significantly greater displacement thanthe displacement corresponding to amplitude A of topography-trackingmotion 202. However, coarse motion 206 is typically limited in itsresolution along the z-axis and is thus not useful for tracking thetopography of surface 108 at high speed. In certain situations, such aswhere the topography changes beyond the range of actuator 110, thecoarse z-axis adjustment provided by additional mechanical systems 112may be used to reposition cantilever 102 or sample 106 to operateactuator 110 within its displacement range.

In a related type of embodiment, actuator 110 can be utilized to providethe oscillation mode motion 204 or the coarse z-direction motion 206. Asin the embodiment described above, the oscillation motion 204 providedby actuator 110 differs from the topography-tracking motion 202 in thatthe oscillatory motion 204 is in response to a narrowband driving signaland has a relatively smaller displacement. The coarse z-axis motion 206provided by actuator 110 is distinguished from the topography-trackingmotion 202 in that the coarse z-axis motion 206 has a substantiallylimited bandwidth. Thus, the coarse z-axis motion cannot be used totrack the topography of surface 108 at the higher scan rates supportedby topography-tracking motion 202.

In one type of embodiment, the actuators for the x, y and z axes arepiezoelectric stacks. In related embodiments, the actuators can alsoemploy any number of alternative actuation technologies, including butnot limited to, other piezoelectric devices, actuators formed fromelectrostrictive, magnetorstrictive, electrostatic, inductive, and/orvoice coil drive mechanisms and other actuators that generate a motionin response to an input signal.

FIGS. 3A-3D illustrate various example configurations for driving apiezoelectric stack-type Z-axis actuator 302 coupled to cantilever 304with topography-tracking signal 306, oscillation mode signal 308, andcoarse z-axis adjustment signal 310 to produce, respectively, motions202, 204, and 206. In the arrangement of FIG. 3A, topography-trackingsignal 306 is applied in series with oscillation mode signal 308 toproduce a superposition of these signals. In FIG. 3B, coarse z-axisadjustment signal 310 is applied to the entire piezoelectric stack 302,while topography-tracking signal 306 is applied to a smaller subset ofstack 302. Oscillation mode signal 308 is applied to an even smallerportion of stack 302. Application of these signals to correspondingdifferent numbers of piezoelectric elements provides desirabledisplacement scaling, resolution scaling, and loading scalingcharacteristics. In the embodiment of FIG. 3C, topography-trackingsignal 306 and oscillation mode signal 308 are differentially amplifiedin driving signal amplifier 312. In the arrangement of FIG. 3D,topography-tracking signal 306 is applied in common mode withpiezoelectric stack 302, while oscillation mode signal 308 is applieddifferentially across a certain portion of stack 302. These exampleconfigurations of FIGS. 3A-3D are merely illustrative of theavailability of different driving arrangements, and should not be takenas an exhaustive presentation of the extent of possible variants.

Referring again to FIG. 1, SPM system 100 has a control system,including monitor 114 and controller 116 that controls motion 202 totrack surface 108. The probe-sample interaction is monitored by monitor114. In one embodiment, monitor 114 utilizes an optical system in whicha laser beam deflection system measures deflection of cantilever 102. Inan oscillating mode embodiment, changes in the probe-sample interactioncan be detected by observing a change in the oscillationcharacteristics, such as in a shift in phase offset of the oscillationdue to a change in resonance characteristics from the probe-sampleinteraction.

Monitor 114 produces signal 115 indicating the probe-sample interaction,and feeds signal 115 to controller 116, which compares it against asetpoint signal representing an amount of probe-sample interaction to bemaintained. Controller 116 produces probe positioning signal 117, whichis input to analyzer 118 as a representation of the topology of surface108. In the embodiment depicted in FIG. 1, monitor 114 and controller116 provide a feedback control topology. In other embodiments that arealso supported within the scope and spirit of the invention, other typesof control topologies are possible. For example, a feed forward controlsystem, or a model-based control system, may be utilized.

Amplifier 120 amplifies probe positioning signal 117 to producecantilever driving signal 121. In one example embodiment, amplifier 120is implemented with discrete and integrated electrical/electroniccomponents on a single circuit board. In other embodiments, the circuitof amplifier 120 spans a plurality of interconnected circuit boards, ora plurality of separate interconnected enclosures. Amplifier 120includes an internal feedback network 122 and load isolating impedance124.

As described above, SPM manufacturers need solutions for facilitatinghigh-speed measurement while providing good measurement sensitivity.This calls for small and thin cantilever probes that have high resonancefrequencies for fast operation, with soft spring constants for increasedsensitivity. Accordingly, aspects of the invention are directed to a newtype of SPM cantilever probe and related methods of fabrication, as wellas to various related features for facilitating high production yieldswith very tight tolerances.

FIG. 4A is a schematic diagram illustrating some of the major parts ofan exemplary cantilever probe 400 according to aspects of the invention.These parts are not drawn to scale. Cantilever probe 400 is composed ofa base portion, or handle, 402. Handle 402 is so named because it isused to handle the cantilever probe. In some embodiments, the otherparts of cantilever probe 400 are so small that they are not visiblewithout substantial magnification. Handle 402 is generally made from arelatively large piece of bulk material such as silicon. Cantilever arm404 has a proximal end 404 a and a distal end 404 b. Cantilever arm 404is made from one primary layer of material, generally silicon, orsilicon nitride, though other materials can certainly be used. In someembodiments, a metallic coating is applied to one side of cantilever arm404 to help with optical reflectivity for use with laser beam deflectionmeasurement arrangements, though certain materials may be sufficientlyreflective at the optical wavelength of interest to obviate thisfeature. Structurally, the material from which cantilever arm 404 isformed has a portion 404 c that is situated over, and is attached to,handle 402, though this portion does not function as a cantilever perse.

At the distal end 404 b is situated the probe tip 406 with apex 408.Probe tip 406 can be formed from silicon in a preferred embodiment,though other suitable materials can be used in other embodiments. In oneembodiment, the probe tip 406 is a pyramidal structure. In anotherembodiment, probe tip 406 has the profile of a right triangle that isaligned with the very end of the cantilever arm 404 such that the apex408 is situated at the distal end of cantilever arm 404. This lattertype of probe tip, called a visible tip, is not actually visible withoutmagnification, and is not visible when the cantilever probe is beingused with a SPM; rather, operators are able to infer its positionknowing that the apex 408 is aligned length-wise with the end of thecantilever arm 404.

In a related embodiment, cantilever arm 404 includes shoulder 410, whichis a wider portion of cantilever arm 404 than narrower neck portion 412.The shoulder 410 is situated at the proximal end over the handle 402.Shoulder 410, by virtue of its much greater width than neck portion 412,has a substantially greater spring constant. In one embodiment, shoulder410 provides no more than a trivial effect on the overall springconstant of cantilever 400, such that the effective length of cantileverarm 404 is measured from the shoulder-neck interface rather than fromthe cantilever arm-handle interface as in a conventional cantileverprobe.

FIG. 4B is a diagram illustrating the beneficial functionality ofshoulder 410. In this diagram, a small, thin cantilever is shown incross-sectional view without a shoulder. The thickness of handle 402 isaffected by the amount of etching performed on it, which is difficult tocontrol precisely. Therefore, some variation in thickness is seen, asindicated by lines 430 and 432. In the case of silicon as the materialfor handle 402, when cantilever 404 is released, handle 402 forms anangle of 125.3 degrees with cantilever arm 404 due to the materialremoval following the crystalline structure of silicon. This results ina variation of effective cantilever arm length indicated at 434. Inconventional cantilever probes, this length variation is negligiblegiven the relatively larger dimensions. However, in the scaled-downcantilever probe, this length variation can significantly affect thespring constant defined by

${k = \frac{E\; w\; t^{3}}{4\; L^{3}}},$

Where E is Young's modulus, w is the cantilever arm width, t is thecantilever arm thickness, and L is the cantilever arm length.

The use of wide shoulder 410 entirely obviates this problem, providedthat the amount of protrusion of shoulder 410 beyond the periphery ofhandle 402 in the distal direction exceeds the potential lengthvariation 434. In one particular embodiment, the shoulder width is atleast 10 times the width of the neck portion 412. In other embodiments,the ratios is of shoulder-to-neck width are 25:1, 50:1, and 100:1 ormore.

Referring still to FIG. 4A, in a related embodiment, at the distal end404 b is situated a wider portion in the form of a paddle face 414.Paddle face 414, in various embodiments, can be circular, elliptical,square, triangular, or any suitable shape. Paddle face 414 provides alarger surface area than what is available from the narrow neck portion404 for providing a laser beam target in the laser beam deflectionsystems of SPM instruments. In one embodiment, the paddle face 414 has agenerally round profile with a diameter in the range of 2-4 microns. Itshould be noted that paddle face 414 may be entirely omitted for systemswhich use a sufficiently small laser spot, or for SPM systems that donot use optical means for determining cantilever deflection. In variousembodiments, the length of the neck portion 404 of the cantilever arm islonger than a diameter of paddle face 414. In other embodiments, theneck portion 404 is shorter than the diameter of paddle face 414. Amongvarious embodiments, the relative dimensions of the neck portion andpaddle face can vary depending on the desired spring constant, desiredresonance frequency, material properties, laser spot size, and otherparameters. In one example embodiment, the ratio of paddle face diameterto neck portion width is at least 2:1. In a related embodiment, theratio is between 2:1 and 4:1. In another example embodiment, the ratioof paddle face diameter to cantilever length is between 2:1 and 1:5.These ratios and ranges are exemplary and are not to be construed aslimiting unless specifically called out in the claims.

According to various embodiments, cantilever probe 400 has dimensionssmaller than 30 microns in effective length (i.e., measured from theshoulder-neck interface), 15 microns in width, and 300 nm thickness. Inone particular embodiment the effective length is between 5 and 30microns, the maximum width is between 2 and 15 microns, and thethickness is between 30 and 300 nm. In a more refined embodiment, thethickness is between 20 and 200 nm. In a further embodiment, thethickness is between 30 and 100 nm. In a further embodiment, thethickness can be made thinner than 30 nm. One particular embodimentprovides a length of 8 microns, a maximum width of 2 microns, and athickness of between 30 and 300 nm. Notably, these dimensions areachieved according to aspects of the invention using wafer-scale, i.e.,batch, processing in which patterning and etching techniques are appliedto simultaneously produce a plurality of cantilever probes in the batchwith size dimensions such as those given above, as well as tighttolerance (e.g., +/−5% thickness variation) at a production yield(defined as non-breakage of cantilevers at the specified length, widthand thickness dimensions during processing) of 90% or better. In termsof cantilever performance, according to one embodiment, the springconstant of the cantilever arm is between 0.1 and 1 N/m and theresonance frequency is between 100 kHz and 10 MHz. In a relatedembodiment, a cantilever probe is produced by a batch process with aspring constant of between 0.1 and 1 N/m with a resonance frequency ofbetween 1 MHz and 10 MHz.

In one aspect of the invention, a multilayered, wafer-scale structure isfabricated initially, using a tight-tolerance process that is, fromwhich a plurality of cantilever probes may be formed using subsequentbatch operations. FIGS. 5, 6A, and 6B illustrate exemplary multilayerstructures as cross-sectional views (not to scale). These exemplarystructures have a common arrangement: a first device layer, a handlelayer, and a second device layer situated therebetween, with separationlayers between the handle and second device layers, and the first andsecond device layers. The first device layer is to be formed into aprobe tip, whereas the second device layer is to be formed into thecantilever arm. In the embodiments detailed below, the handle and seconddevice layers are silicon, while the first device layer can be siliconor low-pressure chemical vapor-deposited (LPCVD) silicon nitride film,also referred to herein as low-stress nitride, with the separationlayers being silicon dioxide, also referred to simply as oxide. Itshould be appreciated, however, that other suitable materials may beused in conjunction with the layered techniques of embodiments of theinvention.

Turning first to FIG. 5, a multilayer structure 500 includes a siliconsubstrate 502 as the first device layer, followed by a first separationlayer 504 made from oxide, a second device layer as the interior layerformed from LPCVD nitride, followed by second separation layer 508, thena second silicon substrate 510 as the handle layer. The separationlayers 504 and 508 are differentially etchable with respect to each ofthe first and second device layers 502 and 504, respectively, and thehandle layer 510. The term differentially etchable implies that onelayer can be etched at a much higher rate than the other layer such thatthe slower-etch-rate material can act as an etch stop layer. Typically,differentially etchable materials have an etch rate ratio of at least50. Depending on the layer material and etchant being employed, thedifferential etch ratio can be on the order of 100, 1000, or more. Inone type of embodiment, the relative thicknesses of adjacentdifferentially-etchable layers are formed to account for thedifferential etch rates that the layer from which materials is strippedand the layer to serve as an etch stop to preserve other layers will beexposed to in subsequent processing, such that the material can bestripped fully, while the etch stop layer remains (albeit somewhatreduced due to the etching). Accordingly, with the multilayer structure,the device layers are protected throughout processing such that theirthickness is unaffected by the duration of etching operations. Thisenables very tight control with small tolerances for the layerthicknesses.

In FIGS. 6A and 6B, the interior layer 606, i.e., the second devicelayer, i.e., the layer to be formed into the cantilever arm, is silicon.Thus, these structures are layered as follows: first device layer 602(silicon), first separation layer 604 (oxide), second device layer 606(silicon), second separation layer 608 (oxide), and handle layer 610(silicon). FIG. 6B includes a pad of conductive material 612, such aspolysilicon, to be situated under the probe tip to be formed from thefirst device layer 602 in subsequent operations. Conductive pad 612provides an electrical connection through the first separation layer 604for use with conductive probes (the probes can be made conductive byusing highly doped silicon for the first and second device layers).

Turning now to FIGS. 7A-7E, an exemplary process of formation ofmultilayer structure 500 is illustrated. In FIG. 7A two separatesubstrates, such as silicon wafers, 502′ and 510′ are prepared. Next, inas illustrated in FIG. 7B, oxide layers 504 a, 504 b, 508 a, and 508 b,is thermally grown on the surfaces. Next, as illustrated in FIG. 7C,LPCVD nitride 506 a and 506 b is deposited over oxide layers 508 a and508 b, respectively. Although nitride layer 506 b is not needed in thisembodiment, there is no need to prevent its deposition because it isremoved incidentally with subsequent steps. As depicted, nitride layers506 a and 506 b are deposited over the handle layer wafer; though itwill be readily appreciated that it could be applied to the first devicelayer substrate instead, or in addition to the handle substrate. Next,layers 506 a and 504 b are polished, and bonded, as illustrated in FIG.7D. Bonding can be achieved with fusion bonding or Anodic bonding (forSilicon to Oxide bonding). Next, as illustrated in FIG. 7E, the firstdevice layer substrate 502′ and handle layer substrate 510′ are groundand polished to their target thicknesses using a chemical-mechanicalpolish (CMP) process or other suitable operation. The resulting layersare silicon device layer 502 and silicon handle layer 510.

FIGS. 8A-8E illustrate a process of forming multilayer structure 600 aaccording to various embodiments. As depicted in FIG. 8A, three siliconsubstrates 602′, 606′, and 610′ are used as the starting point.Separation layers 605 a-f are then created as depicted in FIG. 8B. Inone embodiment, separation layers 605 a-e are oxide layers that arethermally grown. In another embodiment, separation layers 605 a-e arenitride film that is deposited using a suitable process. In anotherembodiment, some of separation layers 605 a-e can be oxide, while othersare nitride.

Next, as illustrated in FIG. 8C, coated substrates 606′ and 610′ arepolished at their interface (between layers 605 d and 605 e) and bondedtogether using a fusion bonding or anodic bonding process, formingburied separation layer 608. As depicted in FIG. 8D, next, the seconddevice layer 606 is formed by grinding and polishing substrate 606′ tothe target thickness. This target thickness is carefully controlledusing suitable process parameters such that a very thin, e.g., 300-nm orsmaller, thickness is attained within the target tolerance. Next, theinterface between first device layer substrate 602′ (coated withseparation layer 605 b) and interior (second device) layer 606 ispolished, and the layers are fusion or anodically bonded as depicted inFIG. 8E forming buried separation layer 604. The top and bottom surfacesare then ground and polished to their target thickness, removing layers605 a and 605 f in the process.

FIGS. 8E′-8H′ illustrate a variation of the processing of FIGS. 8D-8E toform multilayer structure 600 b. The processing illustrated in FIG. 8E′continues from the state shown in FIG. 8D, in which interior (seconddevice) layer 606 is ground and polished to its target thickness. A newseparation layer 615 from either oxide or nitride is grown or deposited,respectively, over layer 606. Next, as illustrated in FIG. 6F′, aportion of the new separation layer 615 on the top surface of layer 606is removed using photolithography and selective etching to create a voidin patterned layer 615′. Then, a layer of polysilicon 612′ is depositedsuch that a portion of the polysilicon fills the void in patterned layer615′. Next, as depicted in FIG. 8G′, the polysilicon 612′ is patternedto remove all of the polysilicon except for portion 612 filling in thevoid in layer 615′. Polysilicon portion 612 thus forms a conductive pad.

Next, the first device layer substrate 602′ is ground and polished toits target thickness, removing layer 605 b in the process, and the topsurface of layer 615′ and conductive pad 612 is polished to create acomposite separation layer 614 with integrated pad 612. As depicted inFIG. 8H′, and the layers are fusion or anodically bonded, forming buriedseparation layer 614. Finally, the top and bottom surfaces are groundand polished to achieve the target thicknesses of first device layer 602and handle layer 610.

Turning now to processing steps for fabrication of cantilever probes,FIGS. 9A-9N illustrate an exemplary process according to one embodimentfor batch fabrication of a silicon-on-insulator (SOI) cantilever probestarting with multilayer structure 500. As depicted in FIG. 9A, oxidelayers 902 a and 902 b are thermally grown over the top and bottomsurfaces of multilayer structure 500. Next, nitride is deposited using aCVD process to create nitride layers 904 a and 904 b. Next, in FIG. 9B,photoresist is applied over the top surface and patterned to create mask906. Mask 906 is used in subsequent steps in formation of the probe tip.

Next, in FIG. 9C, layers 904 a and 902 a are selectively etched using adry etch process to remove all of the nitride and oxide material exceptfor the material masked by the photoresist 906. This leaves structures908 a and 910 a. Next, as depicted in FIG. 9D, photoresist 906 isstripped, and silicon layer 502 is anisotropic etched using a chemicalthat attacks silicon at a much higher rate than oxide, such as potassiumhydroxide (KOH) or tetramethylammonium hydroxide (TMAH), for instance.The result is a pointed tip structure 922 formed from the first devicelayer 502 without affecting the second device layer 506 or the handlelayer 510. Thereafter, as depicted in FIG. 9E, oxide 924 is thermallygrown over the tip structure, which sharpens the tip.

Next, as depicted in FIGS. 9F and 9G, which are cross-sectional views ofthe same structure rotated 90 degrees, photoresist mask 916 is appliedto protect the tip 922 and parts of layers 504 and 506 during patterningof the cantilever arm. Notably, the cantilever arm is patterned beforeit is released. In other words, the handle layer 510 still supports thecantilever arm at this stage, making a more robust structure thatundergoes the cantilever patterning process. FIG. 9H is a top viewdiagram illustrating the cantilever pattern, including shoulder portion930, neck portion 932, paddle face portion 934, and tip 936.

Next, as depicted in FIG. 9I, nitride protective layer 940 is appliedover the oxidized tip structure, and a backside lithography process 942is performed to remove portions of layers 902 b and 904 b. The remainingmaterial of layers 902 b and 904 b act as a mask during the subsequentetching 944 of handle layer 510 to release the cantilever arm, asdepicted in FIG. 9J. Next, as depicted in FIG. 9K, nitride layers 940and 904 b are selectively etched at 946 using a suitable etchant such asphosphoric acid (H₃PO₄), for example. Oxide layers 504 and 508 act as anetch stop to protect interior nitride (second device) layer 506 fromthis etchant.

As illustrated in FIG. 9L, the exposed oxide layers 504 and 508 areremoved at 948 using a suitable etchant that does not attack silicon ornitride at the same rate, such as hydrofluoric acid, for example. Theremaining structure includes remaining handle layer 510′, the protectedand non-removed portion 508′ of oxide layer 508, and the un-removedportion 504′ of oxide layer 504 beneath the tip 922.

FIG. 9M is a top-view diagram illustrating the resulting structure andpatterned cantilever arm. Handle 950 is the large block of silicon. Thecantilever arm has a shoulder portion 952 that protrudes in the distaldirection beyond the periphery of the handle 950 by an amount A, and hasa width B that far exceeds the width of the cantilever neck portion 954.Thus, the effective cantilever arm (as far as its spring constant isconcerned) is composed of neck portion 954 and paddle face 956, on whichthe probe tip 958 is situated. The neck portion 954, being narrow,provides a more sensitive cantilever. Paddle face 956 is useful forproviding a target for the laser system in SPM instruments, whereneeded. Otherwise, the paddle face 956 may be omitted for an even fastercantilever design. Finally, as depicted in FIG. 9N, a metal film 960 isdeposited on the bottom side to enhance optical reflectivity whereneeded.

Turning now to FIGS. 10A-10O, a batch process is described according toa related embodiment for forming a SOI cantilever probe with aself-aligned probe tip that is aligned at the distal end of thecantilever arm, also referred to herein as a visible probe tip.Referring to FIG. 10A, the process begins with the multilayer structure500. Oxide is grown using a thermal process to create oxide layers 902 aand 902 b as before, except this process is followed by selectiveremoval of the top oxide layer 902 a while leaving bottom side oxidelayer 902 b. Next, nitride layers 904 a and 904 b are deposited usingLPCVD.

FIGS. 10B-1 and 10B-2 respectively illustrate cross-sectional and topviews of the next processing steps of photolithography in whichphotoresist layer 1002 is applied and patterned as shown. Various othercantilever arm profiles may be used instead, in which case thepatterning shape will vary accordingly. For example, the paddle shapedescribed above (or variations thereof) may be used in a relatedembodiment.

The photoresist 1002 acts as a mask for etching nitride layer 904 a intopatterned nitride layer 904 a′ using a reactive ion etch (RIE) process,as depicted in FIGS. 10C-1 and 10C-2 in cross-sectional and top views,respectively. FIGS. 10D-1 and 10D-2 illustrate the next step of etchingthrough the first device layer 502 using a high aspect ratio dry etchsuch as DRIE. This creates a vertical surface 1020 along the peripheryof the remaining material in layer 502.

FIG. 10E illustrates the next operation in which oxide layer 504 andnitride layer 506 a are stripped using a RIE process. The RIE processhas no automatic stop, so incidental to this operation, some or all ofoxide layer 508 and some of handle layer 510 may be stripped by RIE.These incidentals are not important at this stage, however, becausethese portions of handle layer 510 and oxide layer 508 will eventuallybe removed in subsequent processing.

Next, as depicted in FIG. 10F, the photoresist 1002 is stripped andvertical oxide 1004 is thermally grown on the orthogonal surface 1020.Next, as depicted in cross-sectional and top views in FIG. 10G-1 and10G-2, the nitride layers 904 a′ and 904 b are removed using a selectiveetch process 1005, and the top side oxide is patterned to leave a smallportion along the orthogonal surface 1020 at the very distal end of whatwill be the cantilever arm.

Thereafter, as illustrated in FIG. 10H, silicon layer 502 is selectivelyetched at 1006 to form probe tip 1008. Probe tip 1008 includes anorthogonal surface on which oxide layer 1004 is grown. This orthogonalsurface also includes the apex of the probe tip. Next, thermal oxide1010 is grown on the other surfaces of probe tip 1008, as illustrated inFIG. 10I. After that, LPCVD nitride layers 1012 a and 1012 b aredeposited on the top and bottom, as illustrated in FIG. 10J. Next,bottom side nitride 904 b and oxide 902 b are patterned and etched usinga photolithography process 1014 to produce cut-away layers 902 b′ and904 b′, as illustrated in FIG. 10H. FIG. 10L represents severaloperations of etching handle 510′, then stripping the nitride layers1012 a and 1012 b′, stripping oxide layer 902 b′, oxide layer 1010,oxide layer 504, and oxide layer 1004. Finally, as illustrated in FIG.10M, a metal layer 950 is applied if needed. FIG. 10N is a perspectiveview diagram illustrating visible probe tip 1008 at the free end of thecantilever arm. FIG. 10O is a scanning electron microscope (SEM) imageshowing an actual visible tip formed using the above process having aheight of 11.5 microns.

Turning now to FIGS. 11A-11L, an exemplary batch process illustratingoperations for fabricating a cantilever probe having a very thin siliconcantilever arm and silicon probe tip is described according to oneembodiment. The process starts with multilayer structure 600 a. Similarto the steps described above for a SOI probe, oxide layers 902 a and 902b are thermally grown, then nitride layers 904 a and 904 b aredeposited, as depicted in FIG. 11A. FIGS. 11B-11C illustrate theoperations for patterning an etch mask for forming the silicon probe tipusing photoresist 906, and forming tip forming masks 908 a and 910 a.FIG. 11D illustrates the next operation of etching the first devicelayer to form tip 922. Next, as illustrated in FIG. 11E, the tip isthermally oxidized and thereby sharpened further.

FIGS. 11F and 11G illustrate the patterning operations of the cantileverarm using photoresist 916. To mask a portion of layers 604 and 606, aswell as the tip, for selective removal of extraneous material fromlayers 604 and 606. These operations are similar to those performed forthe SOI cantilever fabrication, except that the interior layer 606 inthis process is silicon; accordingly, a suitable selective etchant forsilicon is used to etch the cantilever arm. The cantilever may bepatterned to have the paddle-shaped profile as illustrated in theexamples above according to certain embodiments.

Next, as depicted in FIG. 11H, a protective layer 940 of nitride isdeposited over the top side, and photolithography 1142 is performed onthe back-side to strip away portions of the nitride and oxide layers 904b and 902 b, respectively. Next, as illustrated in FIG. 11I, the handlelayer is stripped using the remaining portions of layers 902 b and 904 bas an etch mask, and the cantilever arm is released. Next, as depictedin FIGS. 11J and 11K, the nitride layers 940 and 904 b are strippedusing selective etch process 1146, and the oxide layers 902 b and 924are stripped using selective etch 1148. Next, as shown in FIG. 11L,metal layer 1150 can be deposited if needed.

FIGS. 12A-12L illustrate an exemplary batch process for forming aconductive cantilever probe starting with the multilayer structure 600 baccording to one embodiment. As depicted in FIG. 12A, oxide layers 902 aand 902 b are thermally grown, and nitride layers 904 a and 904 b aredeposited. As illustrated in FIG. 12B, photoresist 906 is deposited overconductive pad 912. These will not be in perfect alignment, but so longas there is sufficient overlap, the probe tip to be formed will beelectrically connected to the cantilever arm. FIGS. 12B-12L illustratethe remaining operations of the process, which are similar to those forforming the non-conductive cantilever probes of FIGS. 11B-11L.Accordingly, FIGS. 12B-12D illustrate masking and etching of the probetip; FIG. 12E illustrates growth of oxide 924 to sharpen the probe tip922, FIGS. 12F and 12G illustrate patterning of the cantilever arm usingphotoresist mask 916 and selective etch operations 1202; FIGS. 12H-12Iillustrate deposition of protective nitride layer 940 and back-sidelithography 1242 to prepare the etch mask for etching the back-sidenitride and oxide layers, and etch operation 1244 to form the handle andrelease the cantilever arm. In operation 1246 as depicted in FIG. 12J,the top and bottom nitride layers are stripped using a selective etch.In operation 1248 shown in FIG. 12K, the remaining top and back sideoxide layers are stripped. Finally, in FIG. 12L, metal film 1250 isdeposited on the back side if needed.

Notably, the processes detailed above produce a plurality of cantileverprobes simultaneously, enabling mass production of such probes at lowunit cost. At the same time, the processes can produce adramatically-improved cantilever probe having very small dimensions attight tolerance, meaning that the cantilever probe is usable with veryhigh-speed SPM equipment by virtue of having a high mechanical resonancepoint, while maintaining a high degree of flexibility (i.e., low springconstant), thereby maintaining high sensitivity to interactions withsamples being examined.

Certain embodiments of cantilever probes produced by these batchprocesses have additional features to support uniformity withinfabricated batches, including the use of shoulder portions formed fromthe cantilever arm device layer, which obviate any length variation dueto variation in thickness of the handle layer. Moreover, certain relatedembodiments include a cantilever arm profile that permits the formationof a very narrow neck portion for added sensitivity while providing apaddle face that can be more easily targeted by a laser system fortracking the movement and deflection of the free end of the cantileverprobe.

Additionally, cantilever probes according to certain embodiments caninclude a visible tip having its apex situated in alignment with thedistal end of the cantilever arm.

The embodiments detailed above are intended to be illustrative and notlimiting. Additional embodiments are within the claims. In addition,although aspects of the present invention have been described withreference to particular embodiments, those skilled in the art willrecognize that changes can be made in form and detail without departingfrom the scope of the invention, as defined by the claims.

Persons of ordinary skill in the relevant arts will recognize that theinvention may comprise fewer features than illustrated in any individualembodiment described above. The embodiments described herein are notmeant to be an exhaustive presentation of the ways in which the variousfeatures of the invention may be combined. Accordingly, the embodimentsare not mutually exclusive combinations of features; rather, theinvention may comprise a combination of different individual featuresselected from different individual embodiments, as will be understood bypersons of ordinary skill in the art.

Any incorporation by reference of documents above is limited such thatno subject matter is incorporated that is contrary to the explicitdisclosure herein. Any incorporation by reference of documents above isfurther limited such that no claims that are included in the documentsare incorporated by reference into the claims of the presentApplication. The claims of any of the documents are, however,incorporated as part of the disclosure herein, unless specificallyexcluded. Any incorporation by reference of documents above is yetfurther limited such that any definitions provided in the documents arenot incorporated by reference herein unless expressly included herein.

For purposes of interpreting the claims for the present invention, it isexpressly intended that the provisions of Section 112, sixth paragraphof 35 U.S.C. are not to be invoked unless the specific terms “means for”or “step for” are recited in a claim.

1-20. (canceled)
 21. A method for constructing a cantilever for use witha scanning probe microscope (SPM), the method comprising: (a) obtaininga multilayer structure comprising an upper substrate, a lower substrate,an interior layer, a first separation layer, and a second separationlayer, wherein the first separation layer is situated between the uppersubstrate and the interior layer, the second separation layer issituated between the lower substrate and the interior layer, and whereinthe first and the second separation layers are differentially etchablewith respect to the first and the second substrates, respectively andwith respect to the interior layer; (b) selectively removing materialfrom the first substrate to form a probe tip structure having a sharppoint facing away from the first separation layer and to expose a topsurface of the first separation layer; (c) after (b), selectivelyremoving material from the first separation layer to expose a topsurface of the interior layer; (d) selectively removing material fromthe second substrate to expose a bottom surface of the second separationlayer; (e) after (d), selectively removing material from the secondseparation layer to expose a bottom surface of the interior layer; (f)selectively removing material from the interior layer to pattern acantilever arm therefrom.
 22. The method of claim 1, wherein (f) isperformed after (c) but before (d).
 23. The method of claim 1, wherein(f) is performed after (e) but before (b).
 24. The method of claim 1,wherein selectively removing material involves applying a differentialetchant that removes corresponding material that is the subject of thatselective etch while leaving other layers substantially unaffected. 25.The method of claim 1, wherein in obtaining a multilayer structure in(a), the first substrate and the second substrate are each formed fromsilicon, and wherein in (b) and (d), selectively removing the materialincludes etching only the silicon.
 26. The method of claim 1, wherein inobtaining a multilayer structure in (a), the first separation layer andthe second separation layer are each formed from an oxide, and whereinin (c) and (d), selectively removing the material includes etching onlythe oxide.
 27. The method of claim 1, wherein in obtaining a multilayerstructure in (a), the interior layer is formed from a nitride, andwherein in (f) selectively removing the material includes etching onlythe nitride.
 28. The method of claim 1, wherein in obtaining amultilayer structure in (a), the interior layer is formed from silicon,and wherein in (f) selectively removing the material includes etchingonly the silicon.
 29. The method of claim 1, wherein in obtaining amultilayer structure in (a), the first separation layer includes aportion formed from a conductive material, and wherein forming the probetip structure in (b) includes preventing etching of the first substratematerial situated over the conductive material. 30-61. (canceled)
 62. Acantilever probe for use with a scanning probe microscope (SPM), thecantilever probe formed according to a method comprising: (a) obtaininga multilayer structure comprising an upper substrate, a lower substrate,an interior layer, a first separation layer, and a second separationlayer, wherein the first separation layer is situated between the uppersubstrate and the interior layer, the second separation layer issituated between the lower substrate and the interior layer, and whereinthe first and the second separation layers are differentially etchablewith respect to the first and the second substrates, respectively andwith respect to the interior layer; (b) selectively removing materialfrom the first substrate to form a probe tip structure having a sharppoint facing away from the first separation layer and to expose a topsurface of the first separation layer; (c) after (b), selectivelyremoving material from the first separation layer to expose a topsurface of the interior layer; (d) selectively removing material fromthe second substrate to expose a bottom surface of the second separationlayer; (e) after (d), selectively removing material from the secondseparation layer to expose a bottom surface of the interior layer; (f)selectively removing material from the interior layer to pattern acantilever arm therefrom.
 63. The cantilever probe of claim 62, whereinin the method of forming the cantilever probe (f) is performed after (c)but before (d).
 64. The cantilever probe of claim 62, wherein in themethod of forming the cantilever probe (f) is performed after (e) butbefore (b).
 65. The cantilever probe of claim 62, wherein in the methodof forming the cantilever probe, selectively removing material involvesapplying a differential etchant that removes corresponding material thatis the subject of that selective etch while leaving other layerssubstantially unaffected.
 66. The cantilever probe of claim 62, whereinin obtaining a multilayer structure in (a) according to the method offorming the cantilever probe, the first substrate and the secondsubstrate are each formed from silicon, and wherein in (b) and (d),selectively removing the material includes etching only the silicon. 67.The cantilever probe of claim 62, wherein in obtaining a multilayerstructure in (a) according to the method of forming the cantileverprobe, the first separation layer and the second separation layer areeach formed from an oxide, and wherein in (c) and (d), selectivelyremoving the material includes etching only the oxide.
 68. Thecantilever probe of claim 62, wherein in obtaining a multilayerstructure in (a) according to the method of forming the cantileverprobe, the interior layer is formed from a nitride, and wherein in (f)selectively removing the material includes etching only the nitride. 69.The cantilever probe of claim 62, wherein in obtaining a multilayerstructure in (a) according to the method of forming the cantileverprobe, the interior layer is formed from silicon, and wherein in (f)selectively removing the material includes etching only the silicon. 70.The cantilever probe of claim 62, wherein in obtaining a multilayerstructure in (a) according to the method of forming the cantileverprobe, the first separation layer includes a portion formed from aconductive material, and wherein forming the probe tip structure in (b)includes preventing etching of the first substrate material situatedover the conductive material.